In the currently practiced processes for making acetylene, calcium carbide is charged to a tank containing water. The carbide is dropped onto the surface of the water where it is allowed to react spontaneously. Acetylene gas is withdrawn from the top of the tank while calcium hydroxide and other impurities settle to the bottom of the tank.
The calcium hydroxide product, along with impurities, is discharged, stockpiled and disposed of as a secondary, low-value by-product Ca(OH).sub.2, also known as hydrated carbide lime. In the current acetylene processes, disposal of the hydrated carbide lime by product, which actually weighs more than the calcium carbide imported into the plant, is becoming increasingly difficult to dispose of because of environmental regulations. The most common usage of hydrated carbide lime is to neutralize industrial acid streams for pH control. Hydrated carbide lime is also used for pH control in sewage treatment facilities. Such applications are of low value and transportation over very large distances becomes prohibitively expensive.
According to the Compressed Gas Association, Inc. (publication #CGA-1.5-1991) other applications of hydrated carbide lime are dehalogenation, metallurgical extractions, gas scrubbing, desulphurization, gas manufacture, masonry additives and as a reagent for manufacture of calcium hypochlorite (a bleach) and calcium magnesium acetate (a deicing agent). Except for these latter two commodities which gain much of their value from the importation of reagents (Cl.sub.2 and Mg.sup.+2 /acetic acid, respectively), all of these uses are low value, probably less than about $50/ton in 1995 dollars.
There are many calcium products currently being marketed, most of which are the inorganic or organic salt of a corresponding acid. Calcium salts currently being marketed derive their calcium from some source other than hydrated carbide lime. This is due in large part to the impurities in the hydrated carbide lime, such as metals, slag, minerals and carbon which come from the coal, coke and limestone used in the calcium carbide manufacturing process. These impurities have heretofore rendered hydrated carbide lime unsuitable as a feed stock for the manufacture of many purified calcium products.
One finished calcium product of particular interest is purified precipitated calcium carbonate (PCC), which is salable in a growing market at high value. A process for manufacture of PCC from hydrated carbide lime would enjoy market, environmental and economic advantages over currently practiced technology.